A little late but if this subject is still of interest the following paper from APE may be of interestwhen considering IR to Air Bath:
HOT AIR versus IR and other convection Rework Systems
HOT AIR is the most popularly chosen method of rework in the industry, with many varied configurations of equipment that can be utilized in a manner best suited for many different types of applications.
There are several important differences between the HOT AIR Vision rework systems and those that use IR technology. Primarily, they are: 1. Profiling and soak programming to create a pattern. 2. Temperature control. 3.Repeatability. 4. Controlled Convection. 5. Process time and 6. Ease of use. It is widely accepted in the industry that forced air convection is the preferred method for both rework and production environments. IR ovens are no longer used in production.
An HOT AIR system can be easily programmed to match any profile with up to 16 segments of ramp soak within a pattern. The system can virtually duplicate the original manufacturing profile for maximum safety for both the PCB and components. Accurate profiling with ramp soak is not possible with an IR system. An IR machine programs the percentage of power (light) rather than temperature. This requires a much higher level of operator skill because it is so easy to burn the boards when using IR.
Shadowing is an issue that occurs with IR systems. If the light source does not hit the board evenly then the heat will not be evenly distributed throughout the board and or the component. This can cause a number of problems with BGA�s including land-sliding, bridging and component warping, which can result in open circuits.
The HOT AIR temperature control is extremely accurate (within +/- 25 degrees F). Controlling temperature with an IR system is a hit or miss proposition, too much light and the board will turn black.
Repeatability is another important advantage that HOT AIR has over an IR system. Once the profile is established it is possible to literally rework hundreds of thousands of components without the need for alignment or calibration. The HOT AIR Sniper II placement accuracy is 0.001�. HOT AIR can report that two important HOT AIR accounts stated that after 24/6/300 and about 100,000 placements, no realignment was required.
A recent discussion with a client who was using two IR systems before switching to a HOT AIR machine had these comments about their IR system:
1. The system can't soak - all it can do is ramp and continue ramping. 2. There's no feature in the software or technology for soaking 3. IR heats the component not the air - this sounds an effective method but if the IR thermal couple (TC) is directed either on the component or on a lead/sphere of the component the air has to increase at least 30% over a convection system � usually resulting in scorching. An HOT AIR rework system monitors the air and its relay is switching many times a second controlling the air whereas an IR system reads the component, opens the relay and just lets that air temperature escalate, which can cause damage. 4. The IR TC is not a laser - it's like a torch light - the nearer the object the more focused the beam - the greater the distance the less focused the beam, this principle only heats up the object of focus - it doesn't heat the environment and if the red dot of light is off it's target, which it often is, it isn�t accurate - it will focus its heat where it comes into focus - before or after the subject or to the side - another important point in this is that the actual TC sensor isn't the light you see, it's a dark piece of the IR spectrum that may not be where the light appears to be focused. 5. Because the system is programmed to reach temperature when the object flows and doesn't consider the environment - it's not repeatable. 6. For the system to work the operator has to focus the IR beam and the camera very accurately on the component leads (in the case of a CSP this is very difficult, the operator has to endeavor to direct a TC light spot and a camera on the leads under the CSP and of course it�s impossible to direct these to the center sphere under a die) the programmer has to be extremely cautious and certain program was halted during the set up at exactly the right time and that this set up was in someway repeatable. 7. The same hit and miss procedure must be gone through for each component. 8. Large boards are a problem and the board holder supplied is problematic. 9. The machine is described as flimsy and fiddly - more of a toy in the opinion of the client 10. In a practical reworking environment reworking different components was impossible. 11. IR systems burn or scorch boards because of insufficient power and the necessity of having to focus the heat on the component body or leads unlike a convection system that heats the air replicating the oven environment. 12. IR has warp issues. 13. Different reflective component surfaces (Shadowing) will distort an IR systems ability to accurately recognize the component and maintain a even temperature distribution.
Other comments were: �There is no such thing as repeatability with IR machines. Results on exactly the same type of component and board were vastly different, for example: Four minutes the 1st time and six minutes the 2nd time etc.� It is extremely easy to burn the boards and there is not enough power to rework boards with heavy ground planes. Warping was a serious issue with IR systems.
As for the argument put forward by IR manufacturers against Convection Rework machines, one of the claimed advantages of an IR system over convection hot air is that nozzles are expensive and hot air machines blow off surrounding components. HOT AIR systems are often delivered complete with nozzles and a lifetime nozzle exchange program. In addition nozzles are designed so that you can use the same nozzle for many different types of components. As will be discussed in more detail below. Be sure to select a HOT AIR machine that has a very low air velocity replicating a convection oven, similar to the ovens used in the original production of the board. This airflow is extremely gentle so that the surrounding components are not disturbed during reflow.
Controlled Convection � Power and Velocity in a Lead-Free Environment:
In considering one manufacturer their Optimum Heater Wattage or Power is an advantage HOT AIR has over IR and other rework systems. The feature that gives HOT AIR greater flexibility and ease of use in reworking components and handling lead free environments is the optimum Wattage of the reflow heater coupled with a carefully controlled constant Velocity of the convective air carrying the heater energy. This combination of Power and Velocity has been proven over 20 years to provide an optimum transmission of Joules (calorific) energy so that the component and PCB is reworked without scorching or disturbance.
Since 2003 other convection systems have endeavored to follow a leading supplier and may claim increased power in response to lead free solders that require higher temperatures to reflow, coupling the power and velocity in a carefully controlled manner is crytical in ensuring that lead free and eutectic solders are reflowed using a reflow pattern that is at an optimum temperature in all segments, never having to increase the ramp rate or temperature above that recommended for the component.
The 1200Watt phenomenon (The importance of not �going over the top�)
It is important to note that a few manufacturers of higher power systems use higher air velocity in an effort to increase the life of their heaters and to transmit the energy through smaller focused nozzles. For these systems to operate at the optimum controlled air velocity would burn out the heaters because of the attenuated airflow through the nozzle. Using a power rating for a heater above 1200Watts results in problems for the reworked subject and also the heater components in a machine.
There is another important issue, which we call the �1200Watt Phenomenon� Using a reflow heater above this Wattage has little or no effect on the energy carried in the air flow � all that happens is that the air runs hotter and the airflow must be increased to allow the heaters to remain functioning. Using a heater below 1200Watts results in insufficient power and the same compromises are installed to overcome the problems that are also associated with higher power heaters � namely increased airflow and higher temperatures.
HOT AIR systems replicate the oven environment allowing it�s low velocity to carry the energy to the component environment gently ramping and soaking the component through it�s programmed pattern until it is either removed or replaced using the lowest possible air temperature to reflow the solder.
This design means that regardless of the density and /or relative weight of the PCB, the machine will achieve the reflow with the minimum temperature input. Typical reflow cycles mimic actual convection oven profiles used in production. Machines that use lower wattage heaters (1200Watts) increase the temperature due to the velocity carrying the energy � of course with lead free solders higher temperatures are desired but in a highly focused design high velocity needs to be applied so that the heaters do not burn out � this results in air an temperature much greater than the reflow temperature of lead free solders and the result is scorching and degradation.
Typically a HOT AIR profile will not require any more than 250c in air temperature to achieve reflow on any board. The machine controller itself will store up to 16 different profiles, and each of those profiles can have as many as 16 heating zones for your use. With a minimum of training, even the most complicated reflow profiles can be run at the touch of a button.
Preheat
We have been discussing reflow temperature and primarily a top reflow heater above the component, it is important to briefly mention the necessity of a pre-heater that can either follow a profile or apply a constant temperature evenly under the subject. These pre-heater or bottom heaters are generally of less power and apply a reduced background temperature to thermally blanket the subject and reduce stresses in potential warping.
HOT AIR systems are equipped with Pre-heaters designed either for 110V (maximum 900Watts) or 220V (Maximum 3600Watts) the former for general rework and the latter for larger infrastructure reworking.
Conclusion
The primary focus of HOT AIR from day one has been �Ease of use� as well as a rugged and reliable construction. As IC devices get smaller and more complex, straightforward processes can be documented using the many optional fixtures and tools available using HOT AIR.
It is interesting to note that those companies that operate as a contract rework business, such as Cable and Satellite Set repair operations choose HOT AIR first. These companies repair several million boards per year at various locations where HOT AIR machines will be seen quietly, quickly and efficiently handling the load - keeping their uses ahead in their industry.
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Hello!
I am an Electrical Technician with Filtronic Comtek....
- Jul 16, 2004
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CJ
I would contact my local distributor. Using a Heat Gun is no...
- Jul 24, 2004
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